This invention relates generally to surface acoustic wave devices and more particularly to adjusting the frequency characteristics of a surface acoustic wave device.
As it is known in the art, surface acoustic wave devices such as resonators, delay lines, filters and pressure transducers are used in a variety of applications. Generally, a SAW device comprises at least one transducer, having a set of conductive members, which is disposed on or recessed within an upper surface portion of a piezoelectric substrate.
A new type of package has been developed which hermeticaly seals the SAW device substrate. This new package includes a pair of piezoelectric substrates which are sealed together by a glass frit, such as described in conjunction with a paper entitled "Saw Resonator Frit Bonded Pressure Transducer" by D. Weirauch et al, Proceedings of the IEEE Ultrasonic Symposium, 1979, p. 874, and an article entitled "Long Term Aging, A Mechanical Stability of 1.4 GHz SAW Oscillators" by M. Gilden et al, Proceedings of the IEEE Ultrasonic Symposium, 1980, p. 184, a second type of package is shown in U.S. Pat. No. 4,270,107, Parker et al issued May 26, 1981 entitled "Stabilized Surface Wave Device" and assigned to the same assignee as the present invention.
Although these packages provide improved packaging and handling capabilities for SAW devices, one problem associated with these packages is that during the fabrication of the package, the frequency characteristics of the package SAW device are changed from that of the prepackaged SAW device. For example, for SAW resonators operating around 350 MHz, this frequency shift may average approximately 45 ppm and may have a standard deviation as high as .+-.15 ppm. For other types of devices and devices which are designed to operate at higher frequencies, both the magnitude and standard deviation of this shift increase making accurate setting of the frequency characteristics of the SAW device more difficult. In many applications, accurate frequency settability of .+-.1 ppm would be desirable.
One solution to this problem has been to use external electrical components to adjust the frequency characteristics of the circuits employing the SAW device. The problem with this approach is that this type of external frequency adjustment is very time consuming, relatively expensive since the number of components required in any particular SAW device application is increased, and furthermore, the components themselves may contribute to frequency changes in circuits employing such SAW devices.